This invention relates to a method and device for providing polarization compensation for a polarization sensitive device. This invention further provides a means of compensating for birefringence in a device or waveguide.
Since most optical signals propagating through optical fiber have an arbitrary polarization state, it is preferred that the switching/routing devices or other devices through which these signals propagate, be substantially polarization insensitive. Notwithstanding, although measures are taken in the design of switches, routers, multiplexers and other components to lessen their polarization sensitivity, tests often indicate that required levels of polarization sensitivity are not met. Planar waveguides usually have different propagation constants for TE (transverse electric) and TM (transverse magnetic) waveguide modes and are known to be polarization sensitive. Stated more simply, the response of these waveguides differs for orthogonally polarized light beams. For wavelength multi/demultiplexers, this difference in propagation constants results in a wavelength shift in the spectral response peak or the passband of each wavelength channel. This wavelength shift is sensitive to the design of the planar waveguide, and can be as large as 3 nm. As WDM systems are being designed towards smaller and smaller channel spacing (from 1.6 nm to 0.8 nm or even less in the future), even a small polarization dependent wavelength shift (e.g. 0.3xcx9c0.4 nm) is of concern.
Two types of integrated wavelength multi/demultiplexers that have been investigated are phased waveguide arrays and grating-on-a-chip spectrometers.
Grating based devices require high quality, deeply etched grating facets. The optical loss of the device depends critically on the verticality and smoothness of the grating facets. However, the size of the grating device is usually much smaller than the phased array and the spectral finesse is much higher due to the fact that the number of grooves in the grating is much larger than the number of waveguides in the phased array. This allows the grating based device to have a larger number of channels available over its free spectral range (FSR) and consequently can be scaled-up easily to high density operation.
In waveguide array based devices, several approaches have been used to reduce the device polarization sensitivity; for example the insertion of a half wave plate in the middle of the waveguides array is described by H. Takahashi, Y. Hibino, and I. Nishi, in a paper entitled xe2x80x9cPolarization-insensitive arrayed waveguide grating wavelength multiplexer on siliconxe2x80x9d, Opt. Lett., vol. 17, no. 7, pp. 499-501, 1992.
One patent which provides a useful solution for polarization compensation in a slab waveguide device, is U.S. Pat. No. 5,937,113 issued Aug. 10, 1999 to He et al., entitled xe2x80x9cOptical grating-based device having a slab waveguide polarization compensating regionxe2x80x9d.
Notwithstanding, the instant invention is believed to provide a more general solution to this problem of compensating for polarization sensitive optical devices, making them less sensitive to the polarization state of light passing therethrough.
The instant invention relies on the use of an in-line fibre grating to equalize the polarization dependence of an optical device. A similar type of blazed grating has been disclosed at OFC 1992, Feb. 4-7, 1992, San Jose Convention Center, San Jose, Calif., in a Tutorial Session (Tutorial TuK, page 132 of the Tutorial Digest) entitled xe2x80x9cFiber-Based Passive Componentsxe2x80x9d presented by Kenneth O. Hill, the applicant on Feb. 4th, 1992.
Another later disclosure of a blazed grating used as a tap is found in U.S. Pat. No. 5,061,032 to G. Meltz et al. which discloses an optical fiber tap that comprises a blazed, chirped refractive index grating selected to redirect light guided in the fiber such that it comes to a focus at a point outside of the fiber. It is an object of Meltz et al. to provide a tap for extracting light from the grating section through the side of an optical fiber.
U.S. Pat. Nos. 5,850,302 and 5,832,156 in the name of Strasser et al. disclose improvements on the invention of Meltz et al. Strasser et al., further elucidate problems with the device of Meltz et al. by stating that xe2x80x9cthe tap of the ""032 patent has some shortcomings. For instance, due to the relatively large (exemplary 22xc2x0) blaze angle that is required to achieve the desired redirection of the light guided in the fiber core to light in space outside of the fiber, the arrangement is subject to undesirable polarization effects, i.e., the fraction of light that is redirected by the grating depends on the polarization of the incident guided light. Whereas for low blaze angles ( less than 10xc2x0) the polarization dependent difference in the amount of redirected light is at most about 0.54 dB, this difference increases rapidly with increasing blaze angle, being about 2.86 dB and about 6.02 dB for blaze angles of 22xc2x0 and 30xc2x0, respectively. Furthermore, as those skilled in the art will appreciate, the fraction of redirected light decreases with increasing blaze angle, for a given index change. See, for instance, T. Erdogan et al., J. of the Optical Society of America-A, Vol. 13(2), p. 296 (1996).xe2x80x9d
Strasser et al., further state that in view of the many important potential uses of an efficient, wavelength-selective fiber tap, it would be desirable to have available a fiber tap that is substantially free of the shortcomings such as the polarization dependence of the above discussed prior art tap. The Strasser specification illustrates the advantage of using blazed gratings having angles of about 15xc2x0 or less.
It is clearly the goal of both Strasser et al., and Meltz et al., to provide a tap that is substantially polarization insensitive and which allows a suitable amount of light to be tapped. Both of these requirements rely on using blazed gratings having small angles, i.e., 15xc2x0 or less.
Tilted or blazed fiber Bragg gratings (FBG""s) have been used to fabricate devices that couple light from guided modes into radiative modes which are highly polarization sensitive at large blaze angles [4, 5]. For an FBG tilted 45xc2x0 with respect to the fiber axis, there is a difference of two orders of magnitude between radiation mode coupling for s-polarized (in the plane of the tilted index modulation) and p-polarized light. [6] Recently an all fiber polarimeter was fabricated using blazed FBG""s. [7]
In contrast, it is an object of the instant invention, to provide a blazed grating in combination with an optical element exhibiting polarization sensitivity, wherein the blazed grating taps a minimal quantity of light therefrom and is substantially of the opposite polarization of the element and therefore is suitable for compensating for the polarization sensitivity of the optical element.
Along with the need to provide polarization compensation in an optical device, there is often the requirement to provide a particular birefringence in order to lessen unwanted polarization mode dispersion. In some instances it may be desired to induce a certain amount of birefringence into the optical circuit. Along with providing a novel solution for lessening polarization sensitivity by providing a blazed grating, this invention further provides a method of providing an induced birefringence thereby providing compensation for polarization mode dispersion (PMD) well as polarization dependent loss (PDL).
Further, with the present invention, PDL compensating tilted FBG""s are fabricated using excimer laser exposure through a zero-nulled order phase mask. For a given phase-matching condition and grating tilt, the overall PDL is controlled through the UV-induced refractive index variation and the grating length.
In accordance with an aspect of the invention, there is provided, an optical system comprising: an optical device for passing light in a predetermined wavelength range, having substantially unequal insertion losses for orthogonal polarizations of light launched into the device and thus has a polarization dependent loss within said wavelength range; a waveguide having a longitudinal optical axis that is connected to said optical device, the waveguide having a blazed grating therein having a blaze angle xcex8 of between 20xc2x0 and 60xc2x0 with respect to the optical axis of the waveguide, the blazed grating exhibiting a polarization dependent loss that is of the same magnitude as the polarization dependent loss of the connected optical device, and the grating being oriented about its optical axis to lessen the total polarization dependent loss of the combination of said blazed grating and said optical device.
In accordance with another aspect of the invention, a method of lessening polarization dependence of an optical device to a band of wavelengths of light passing therethrough is provided, comprising the steps of providing in an optical waveguide, coupled with the optical device a blazed grating, having a blaze angle of between 15xc2x0 and 85xc2x0 and orienting the blazed grating such that a polarization dependence of the device at the band of wavelengths passing therethrough is lessened.
Advantageously, this invention provides a method of further controlling polarization aspects related to the waveguide by irradiating the waveguide in a prescribed manner to induce a birefringence.